The present invention relates to a sintered fuse which is built in, for example, in a solid electrolytic capacitor, in particular, to a sintered fuse which can fuse at a temperature higher than, for example, the soldering temperature and lower than the thermal withstanding temperature of an element to be protected, such as a solid electrolytic capacitor, as well as to a solid electrolytic capacitor which utilizes the fuse.
A conventional solid electrolytic capacitor is formed by electrically connecting, respectively, the positive electrode and the negative electrode of a capacitor element, which is a sintered body of a valve action metal powder, such as a tantalum powder, to the external leads and by covering the periphery with a package made of a resin. And, by interposing, on the negative electrode side, for example, a fuse between the negative electrode of the capacitor element and the external lead, a structure is obtained wherein harmful effects to other parts of the circuit can be prevented even in the case that the capacitor element arises in temperature due to an irregularity of the circuit, or the like, and the temperature of the capacitor element increases excessively so as to be in a short circuited condition.
In the case of a tantalum electrolytic capacitor, for example, the capacitor element made of a sintered body of tantalum powder burns out at approximately 600xc2x0 C. Therefore, it is necessary to cut off the circuit at a temperature lower than that. On the other hand, this type of solid electrolytic capacitor is mounted on a circuit substrate through a solder reflow or a solder flow. The temperature for this mounting operation is approximately 230xc2x0 C. to 260xc2x0 C. and therefore the fuse needs to withstand a temperature higher than that. From such a point of view, conventionally, an Snxe2x80x94Pb alloy (Ag is slightly included in the ratio of Sn:Pb=1:9) is used as a material of the fuse so as to fuse at approximately 300xc2x0 C.
As described above, an alloy including Pb is generally used as a material for a fuse which fuses at approximately 300xc2x0 C. Pb is a heavy metal, however, and the trend today is to ban the utilization of Pb from the point of view of environmental pollution prevention. There is no material which has the same melting point level without the utilization of this Pb and the development of a substitute which fuses at a temperature of approximately 300xc2x0 C. has become an urgent problem to be solved.
The present invention is provided considering such circumstances and the purpose thereof is to provide a fuse which fuses at approximately same level of temperature as that in a prior art without utilizing toxic Pb.
Another purpose of the present invention is to provide a solid electrolytic capacitor utilizing a built in fuse which does not utilize Pb and which fuses effectively.
The present inventor has researched diligently, as described above, in order to obtain a material which fuses at a low temperature, of approximately 300xc2x0 C., without utilizing Pb, and which can substitute for a conventional fuse, and, as a result, it was found that the fine metal powders sintered was melted at a temperature of approximately ⅓ or less of the original melting point of the metal. For example, when powders of 1 to 3 xcexcm of tantalum were sintered in a vacuum, and when heat was applied, the material change was examined by a thermo-scale, and it was found to melt at approximately 600 to 800xc2x0 C., in spite of the original melting point of tantalum of approximately 3000xc2x0 C., and it was found to be able to be utilized as a fuse material even in the case that Pb, of which the melting point is appropriately low, is not utilized.
A sintered fuse according to the present invention comprises: a sintered body of metal powders in a wire form or a plate form.
By having this structure, a material which fuses at approximately 300xc2x0 C. can be obtained by, for example, sintering copper powders. And by forming a wire form of the diameter of approximately 0.1 mm, the substitute is obtained which can be utilized for a fuse, of which the main component is Pb, according to a prior art. Here, due to the conditions at the time of sintering, close contact conditions between powder grains change and the melting temperature is also changed. For example, when the sintering condition is set at high temperature, the adhesion between powders becomes stronger, and exhibits properties that are closer to the original nature of the simple metal so that the melting temperature becomes higher. In addition, in the case that the sintering temperature is low, the sintering will not proceed sufficiently, the meting temperature becomes lower and, in addition, contact resistance value between fine powder grains increases so that heating effect by an excessive current is generated to easily melt.
A solid electrolytic capacitor according to the present invention comprises a capacitor element which has a positive electrode lead and a negative electrode which is formed on the outer periphery surface, first and second external leads to which the above positive electrode lead and negative electrode are electrically connected, respectively, a fuse which is interposed, in either one of the electrical connections, between the above positive electrode lead and the first external lead or between the above negative electrode and the second external lead and a package which covers around the above capacitor element, wherein the above fuse is constructed of a sintered fuse formed of a sintered body of metal powders.